This invention relates to inexpensive process and reactor modifications for the reduction of nitrogen oxides (NOx) emissions from catalytic cracking regenerators. More specifically, this invention relates to operating the regenerator such that up to about 1% carbon monoxide (CO) exits the dense catalyst zone, and to modifications that provide for the introduction of secondary oxygen-containing gas streams and, optionally, shield gas stream or streams into the dilute phase of the regenerator, thereby eliminating the majority of NOx emissions without producing significant CO emission and reducing temperature rise due to afterburn.
In the fluid catalytic cracking (FCC) process, hydrocarbon feedstock is injected into the riser section of a hydrocarbon cracking reactor where it cracks into lighter, valuable products on contacting hot catalyst circulated to the riser-reactor from a catalyst regenerator vessel. As the endothermic cracking reactions take place, the catalyst is covered with coke deposits. The catalyst and hydrocarbon vapors are carried up the riser to the disengagement section of the reactor where they are separated. Subsequently, the catalyst flows into the stripping section where the hydrocarbon vapors entrained with the catalyst are stripped by steam injection, and the stripped catalyst flows through a spent catalyst standpipe and into the catalyst regenerator vessel.
The regenerator vessel is operated as a fluid bed reactor with the catalyst forming a dense phase in the lower section of the reactor and a dilute phase above the dense phase. Air or oxygen-enriched air is introduced through an air grid located in the dense phase near the bottom of the vessel. When the coke-laden catalyst comes in contact with the air the coke is burned forming CO and carbon dioxide (CO2), which, along with the nitrogen in the air, pass upwards through the dense phase, into the dilute phase, and then exits the regenerator. These gases constitute the majority of the flue gas. During the coke combustion process, any nitrogen containing species present in the coke also react with oxygen to form mostly elemental nitrogen (N2) and a small amount of NOx. These species, along with any sulfur oxides (SOx) formed by the combustion of sulfur present in the coke, also travel with the CO/CO2/N2 through the regenerator. The region of the reactor near the air grid, within the dense phase, has a high oxygen concentration that constitutes the oxidizing zone. Away, or downstream from the air grid, as oxygen is depleted, a reducing zone forms, where the CO concentration is significant. The CO continues to react with the remainder of the oxygen to form CO2. In the reducing zone, NOx species also react with CO to form elemental nitrogen. Depending on the concentration of CO and CO2 in this zone, more or less NOx will react.
The catalyst regeneration vessel may be operated in the complete CO combustion mode, which has now become the standard combustion mode, or in partial CO combustion mode. In the complete combustion operation, the coke on the catalyst is oxidized completely to form CO2. This is typically accomplished by conducting the regeneration in the presence of excess oxygen, provided in the form of excess air. The exhaust gas from a complete combustion operation comprises primarily nitrogen, CO2, H2O and excess oxygen, but also contains NOx and SOx.
In the partial CO combustion mode of operation, the catalyst regeneration vessel is operated with insufficient oxygen to fully oxidize all of the coke in the catalyst to CO2. Consequently the coke is combusted to a mixture of CO and CO2. The remaining CO is oxidized to CO2 in a downstream CO boiler. When the regeneration vessel is operated in the partial CO combustion mode, less NOx is produced, and that which is produced reacts with CO in the reducing zone to form elemental nitrogen. Instead, nitrogen species in the coke leave the regeneration vessel as reduced nitrogen species, such as, ammonia and HCN. However, the reduced nitrogen species are unstable in the CO boiler, where they are converted to NOx. Thus the effluent from the CO boiler comprises primarily nitrogen, CO2 and H2O, but also contains NOx and SOx.
Recently, there has been considerable concern about the amount of NOx and SOx being released to the environment in refinery flue gases. It is now the accepted view that most of the NOx present in catalyst regenerator exhaust comes from coke nitrogen, i.e., nitrogen contained in the coke in the form of hetero-compounds, such as, condensed cyclic compounds, and that little or none of the NOx contained in the exhaust gas is derived from the nitrogen contained in the air feed to the regeneration vessel.
Several approaches have been used in industry to reduce NOx in FCC regenerator vessel exhaust gases. These include capital-intensive and expensive options, such as pretreatment of reactor feed with hydrogen, and flue gas post-treatment options, such as Selective Catalytic Reduction (SCR), as well as the use of in-situ FCC catalyst additives. A number of other methods have also been contemplated for NOx reduction, as discussed below.
U.S. Pat. No. 5,268,089 discloses that NOx can be reduced by operating the regenerator xe2x80x9con the brinkxe2x80x9d, i.e., in a region between conventional partial CO combustion operation and complete combustion operation with less than 2 mol % CO in the flue gas. The patent claims NOx reduction by operating in this mode. However, a CO boiler is still required to burn the CO exiting from the regenerator, as is the case in the partial combustion mode of operation. Furthermore, while U.S. Pat. No. 5,268,089 discloses the existence of afterburn as a result of operating xe2x80x9con the brinkxe2x80x9d, a solution to avoid or mitigate the overheating in the dilute phase due to afterburn is not disclosed.
Several patents disclose the reduction of NOx in FCC regenerators by means of promoters, segregated feed cracking, post treatment of exhaust gas, etc. These patents are discussed in detail in U.S. Pat. No. 5,268,089, the disclosure of which is incorporated herein by reference.
U.S. Pat. Nos. 5,705,053, 5,716,514, and 5,372,706 each disclose variations of the basic idea of controlled air addition to flue gas from a regenerator operated in the partial combustion mode, before the CO boiler, to convert part of the NOx precursor species (HCN, NH3) selectively to N2 rather than NOx. Consequently, in the CO boiler, less NOx is generated. In U.S. Pat. No. 5,705,053 an additional catalytic step is suggested for NOx/NH3 reaction. In U.S. Pat. No. 5,372,706, the thermal conversion of NOx precursors is claimed at temperatures between 2000 and 2900xc2x0 F. In U.S. Pat. No. 5,716,514 flue gases are specifically removed from the regenerator and comprise at least 2.5% carbon monoxide. These gases are reacted in a separate turbulent flow reactor. In all of these patents, the secondary air addition is aimed at reacting part of the NH3/HCN formed due to the partial combustion operation.
U.S. Pat. No. 5,240,690 suggests a partial combustion mode of operation and the addition of air to the regenerator off-gas comprising at least 1% carbon monoxide to oxidize NH3/HCN and preferentially produce N2 prior to the CO boiler.
Efforts are continuously underway to find new and improved methods of reducing the concentrations of NOx and SOx in industrial flue gases, such as, FCC regeneration vessel exhaust gases. Notably absent from the prior art is the introduction of secondary oxygen-containing gases, optionally with shielding gases, into the dilute phase of the regeneration vessel, which is primarily operated in a complete combustion mode, whereby the majority of NOx is eliminated, CO is converted to CO2, and the temperature rise due to after burn is controlled.
The present invention provides inexpensive regeneration vessel modifications that significantly reduce NOx emissions by concurrently introducing secondary oxygen-containing gases, optionally with shielding gases, into vessel which is operated in a manner that does not require the use of a CO boiler. The present invention provides means to eliminate the majority of NOx emissions from a FCC regenerator.
The present invention is directed to a process for substantially reducing the emission of nitrogen oxide from a regeneration reactor during the regeneration of a spent catalyst, such as, a hydrocarbon cracking catalyst, having coke deposits thereon, which comprises the steps of:
(a) contacting the spent catalyst with a primary oxygen-containing gas in the dense phase of the reactor, thereby combusting the coke and forming a combustion gas comprising nitrogen oxide and carbon monoxide which further react in said dense phase, thus reducing a majority of the nitrogen oxides to form elemental nitrogen, thereby forming a nitrogen-enriched combustion gas; and
(b) contacting the nitrogen-enriched combustion gas in the dilute phase of the reactor with a secondary oxygen-containing gas, wherein the carbon monoxide is oxidized to form carbon dioxide.
The amount of the primary oxygen-containing gas in step (a) is adjusted so that the nitrogen-enriched combustion gas prior to step (b) comprises up to 1% carbon monoxide. As a result of this process, nitrogen oxide emissions from the regeneration reactor are significantly reduced while the temperature rise due to afterburn in the dilute phase is minimized and controlled by the introduction of a shield gas or heat removal devices.
The present invention also employs one or more nozzles configured to allow the secondary oxygen-containing gas, and, optionally, a shield gas, to be introduced into the dilute phase of the regeneration reactor so as to provide combustion conditions, and control the temperature rise due to afterburn in the dilute phase.
The secondary oxygen-containing gas introduced to the reactor oxidizes the residual CO exiting the dense phase. Steam or water may be added to the secondary oxygen-containing gas stream as a shield gas to assist in the even distribution of oxygen across the regenerator vessel and to reduce the temperature rise in the dilute phase due to the combustion of CO. The location of the one or more nozzles feeding the steam or water is selected such that there is minimal contact of steam with the majority of the catalyst, thereby avoiding catalyst deactivation. The excess heat generated in the dilute phase due to the exothermic CO oxidation may also be removed by other means, such as, for example, with a cooling coil located in the dilute phase.
The present invention may also be configured in such a manner that the secondary oxygen-containing gas is introduced in different stages at different vertical heights in the reactor vessel. For example, a portion of the secondary oxygen-containing gas may be introduced to the reactor at or just above the interface between the dense phase and the dilute phase, prior to introducing the main secondary oxygen-containing gas, as described above. The secondary oxygen-containing gas introduced at the interface is in an amount sufficient to combust the small amount of residual reduced nitrogen species, such as, NH3 and HCN, contained in the combustion gas to form nitrogen oxides, which are subsequently reacted with CO to form elemental nitrogen. The secondary oxygen-containing gas is then staged or introduced to the dilute phase of the vessel at a point downstream in order to perform the process as described above. The secondary oxygen containing gas may be introduced to the interface between the dense and dilute phases of the reactor with one or more nozzles. Additionally, a shield gas may be introduced with the secondary oxygen-containing gas to assist in the even distribution of oxygen across the vessel and to avoid catalyst deactivation by creating a gas barrier between the steam introduced above that point and the catalyst.
In another embodiment of the present invention, to further eliminate the small amount of CO that may escape from the dilute phase of the regenerator vessel, the secondary oxygen-containing gas is staged or introduced to the exhaust flue of the vessel after carrying out the processes as described above. The oxygen-containing gas oxidizes any remaining CO that may be present in the flue gas, thus forming CO2. The secondary oxygen-containing gas may be introduced to the exhaust flue with one or more nozzles. Additionally, a shield gas may also be introduced to assist in the even distribution of oxygen across the exhaust duct and control the temperature rise due to afterburn.
Significant cost savings relative to the large reduction in NOx occur since no separate NOx removal equipment is required downstream. In addition, the process of the present invention is advantageous since it involves minimal modifications to the existing regeneration vessel and associated equipment, compared to installation of a secondary air grid. Furthermore, because there is virtually no CO exiting the reactor, no downstream carbon monoxide boiler is required.
The present invention is also directed to a catalyst regeneration vessel having a dense phase and a dilute phase comprising:
(a) a means for introducing a primary oxygen-containing gas into the dense phase of the regeneration vessel; and
(b) a means for introducing a secondary oxygen-containing gas into the dilute phase of the regeneration vessel.
In one embodiment of the present invention, the regeneration vessel is a reactor. Preferably, a means for introducing a primary oxygen-containing gas to the reactor is an air grid located in the dense phase. In addition, preferably, one or more means for introducing a secondary oxygen-containing gas, and optionally a shield gas, into the reactor is in the form of nozzles, which are located in the side walls and/or the dome of the reactor at various angles and heights. In another embodiment of the present invention, the reactor has a means for staging the introduction of the secondary oxygen-containing gas to the dilute phase of the reactor at or just above the interface with the dense phase. In yet another embodiment of the present invention, the reaction vessel has a means for staging the introduction of the secondary oxygen-containing gas to the exhaust flue of the vessel.